Problem 34 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Arrange the following sulphates of alkaline earth metals in order of decreasing thermal stability: \(\mathrm{BeSO}_{4}\), \(\mathrm{MgSO}_{4}, \mathrm{CaSO}_{4}, \mathrm{SrSO}_{4}\)

Step-by-Step Solution

Answer

The order is: SrSO_4, CaSO_4, MgSO_4, BeSO_4.

Step 1: Understand Thermal Stability

Thermal stability of sulphates generally increases down the group in the periodic table. Alkaline earth metals are in Group 2, and as we move from beryllium to barium, the thermal stability of their sulphates typically increases.

Step 2: Identify the Alkaline Earth Metals

The given sulphates are of the metals beryllium (Be), magnesium (Mg), calcium (Ca), and strontium (Sr). These metals belong to Group 2 (alkaline earth metals) of the periodic table, listed in increasing order of atomic number.

Step 3: Apply Thermal Stability Trend

Based on the trend that thermal stability increases down the group, arrange the given sulphates. The correct order of decreasing thermal stability is following the reverse trend: 1. BeSO_4 (most thermally stable) 2. MgSO_4 3. CaSO_4 4. SrSO_4 (least thermally stable).

Step 4: Write the Answer in Decreasing Order

Now, arrange the sulphates in decreasing order of thermal stability based on the observations: 1. SrSO_4 2. CaSO_4 3. MgSO_4 4. BeSO_4

Key Concepts

Alkaline Earth Metals

Alkaline earth metals are the elements found in Group 2 of the periodic table. These include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Alkaline earth metals are known for their distinctive characteristics:

• They each possess two electrons in their outermost shell.
• These metals are shiny and relatively soft compared to transition metals.
• They readily form cations with a +2 charge when they react.

Despite being referred to as "earth" metals, these elements are found in many different types of minerals on Earth.
Their reactivity increases as we move down the group in the periodic table, meaning that beryllium is the least reactive and radium is the most reactive.
Alkaline earth metals form a variety of compounds, including sulphates like those given in the exercise. These sulphates exhibit different levels of thermal stability depending on which metal they are paired with.

Sulphates

Sulphates are salts or esters of sulfuric acid and contain the sulfate ion, \(\text{SO}_4^{2-}\). When alkaline earth metals combine with sulfate ions, they form what are known as metal sulphates, such as beryllium sulfate \(\text{BeSO}_4\) and magnesium sulfate \(\text{MgSO}_4\).
Metal sulphates can differ significantly in terms of thermal stability.

• Thermal stability refers to how well a compound can withstand high temperatures before decomposing.
• Sulphates of alkaline earth metals like Be, Mg, Ca, and Sr generally show varied levels of thermal stability.
• Understanding these differences is crucial for applications such as material design and thermal processing technologies.

The exercise discussed involves arranging sulphates in a specific order based on their thermal stability properties.

Periodic Table Trends

Periodic table trends provide a predictable pattern of chemical behavior throughout the elements. For alkaline earth metals, one of the key trends observed is the increase in thermal stability of their sulphates moving down the group. This means:

• Starting from beryllium, the sulphates become more thermally stable.
• The trend is influenced by the increasing atomic size and decreasing electronegativity as we move down the group.
• Larger atoms, like strontium, form weaker metal-sulfate bonds, which means less energy is required to break them.

In contrast, smaller atoms like beryllium create stronger bonds, resulting in higher thermal stability.
Understanding these trends is pivotal in predicting and explaining the behavior of chemical compounds, as demonstrated in the exercise with the order of decreasing thermal stability for the sulphates of alkaline earth metals.